This invention relates to improvements in damping plates (also known as heave plates) and, while not so limited, relates particularly to damping plates used in apparatus for converting energy contained in waves on the surface of bodies of water to useful energy.
The use of damping plates, e.g. a horizontally oriented flat plate attached to a submerged vertical spar of a floating oil platform, is generally known. The plates have the effect of increasing the mass and the viscous drag of the members to which they are attached, thus making the members less likely to be driven into destructive movements by excessively high surface waves and winds.
Such damping plates are also useful in certain types of apparatus, known as wave energy converters (WECs), for converting energy in surface waves on bodies of water to useful energy. One type of such WEC comprises a pair of floats which bob up and down in response to passing surface waves. Relative movements between the floats are used for driving an energy converter, for example, an electrical generator connected between the floats for generating electrical or any useful energy.
The amount of energy produced by a given WEC is a function of, among other things, the mass of the floats driven into motion by the passing waves. For increasing the effective mass of at least one of the floats, a practice is to suspend a submerged damping or heave plate from the float. The effective mass of the float from which the damping plate is suspended is increased due to the fact that the acceleration and deceleration of the plate through the water requires movements of some volume of water around the plate. The greater the amount of water moved by the plate, the greater is the increase of the float mass.
The amount of water moved by a given damping plate is a function of the size of the plate. Also, as disclosed in co-pending patent applications, titled “Heave Plate with Improved Characteristics” bearing Ser. No. 11/796,851, and “Wave Energy Converter (WEC) with Heave Plates bearing Ser. No. 11/796,852, assigned to the assignee of the present application, and the subject matter of which is incorporated herein by reference as though fully set forth herein, the addition of a vertically extending structure, e.g. a lip along the edges of the heave plate, significantly increases the amount of water moved by the plate, hence the amount of mass added to the float.
While useful for adding effective mass to a float, a problem associated with the use of damping plates, at least in connection with WECs, is that the viscous drag forces associated with the plates tends to impede relative movements, thus tending to negatively affect power generation. The present invention provides means for minimizing the drag forces typically associated with the use of damping plates of the type described.